tmmlrd:
I have to drive some high power LEDs (using TIP122 darlingtons), for a total current around 2 A on 12 V input. I'm wandering if I can drain this current from the V_in and GND pins of the Arduino Uno board, just connecting a 12 V supply to the Arduino board and using it to power both board and loads
Don't do it.
a) The electrical connection will be rubbish. The Arduino connectors are good for poking wires into for prototyping but not designed for high current.
b) 2A through a thin trace on a circuit board? Not good.
If you really really don't want to connect directly to the PSU then solder a wire to the power connector on the bottom of the Arduino board. There's a great big solder pad there...